1. Field of the Invention
The present invention relates to an AD converter for converting an analog signal to a digital signal. In particular, it relates to an AD converter capable of analog-to-digital signal conversion in a short time.
2. Description of Related Art
Music, images and moving pictures, which have been recorded as analog data, are now being recorded as digital data with high quality and enhanced convenience. An AD converter has been used to convert analog data into digital data. For capturing images and moving pictures, image sensors such as a CCD (charge coupled device) and a CMOS sensor have been used and the need for signal processing devices for converting an analog image output signal into a digital signal is increasing. Also in many sensors used for controlling various devices, an analog output signal is converted into a digital signal to facilitate the processing in the control unit. Thus, the range of uses of the AD converter for converting an analog value to a digital value has been increasing.
Although there are various AD converters of different structures, an AD converter of relatively simple structure has been proposed by Japanese Patent No. 3507800, for example. This AD converter generates an analog reference voltage and performs a comparison between the reference voltage and an input signal voltage while gradually changing the reference voltage. Then, the level of the signal voltage is judged from the value of the reference voltage at a point of time when the magnitude relation between the reference voltage and the signal voltage has changed such that the signal voltage is converted into a digital value.
In this AD converter, as shown in FIG. 18, an analog signal of a selected signal line (φs) which is read and stored in a first period T1 is converted into a digital signal in a second period T2. More specifically, an analog signal (Vs) stored in a sample hold circuit is input to one of input terminals of a comparator and a reference voltage (Vc) which increases with time is input into another input terminal of the comparator, while output data (DATA) of a digital counter to which clocks (CLK) are input increases one by one. The output of the comparator (Vcomp) for comparing the analog signal (Vs) with the reference voltage (Vc) causes a change of state at around a point of time when the analog signal (Vs) and the reference voltage (Vc) agree with each other. The output data (DATA) of the digital counter at the point of time when the change of state occurs is latched. The latched output data (DATA) is a digital signal corresponding to the analog signal (Vs).
With the above-described AD converter, however, the comparison between the signal voltage and the reference voltage is performed successively while changing the reference voltage from the minimum value to the maximum value in the range of every possible signal voltage. Therefore, long time has been taken to convert the analog signal to the digital signal. In order to increase the resolution of a digital value by 1 bit, the number of comparisons must be doubled. Therefore, the long conversion time is almost doubled. Thus, the conventional AD converter involves a problem in the conversion of a digital value with a high bit count.
Further, in an AD converter for converting an analog signal of an image sensor or the like into a digital signal, it is desirable to take a difference between an output value of a dark pixel and an output value of a light-exposed pixel as a signal output such that noise components are removed. For that purpose, there are a method of taking the difference between the analog signals before digital conversion and a method of taking the difference after the digital conversion of the analog signals from the dark and light-exposed pixels. In the latter method, however, the digital conversion must be performed twice on the signals from the dark and light-exposed pixels. Therefore, the analog-to-digital signal conversion takes long time.